The objective of this continuing reaserch is to examine two related questions: (1) the relation between measured intelligence and family size; and (2) the relation between measured intelligence and birth order. The analyses will be based on data from the Wisconsin Longitudinal Study. Our second research paper on the relation between intelligence and family size will focus on the question, "How Intelligence Affects Fertility." The research will include path and other multivariate analyses of how background factors, IQ, and intervening factors affect fertility. The research on the influence of birth order on IQ will be done first in the usual way with between-families data from the full Wisconsin sample. The second and more novel approach will employ within-families data from the Wisconsin sib sample and will use the respondent-sib difference in IQ, indexed by birth orders of respondent and sib, as the dependent variable. This within-family difference automatically controls for many confounding factors, since respondent and sib share the same parents. The second approach therefore promises to give more informative results than the first. The Wisconsin sib sample is well-suited to test the key results than the first. The Wisconsin sib sample is well-suited to test the key prediction of the latest version of the Zajonc-Markus confluence model of how birth order and related family structure variables affect intellectural development. This test of the confluence model, which will also include investigation of sibship size effects, will form a major focus of the continuing research. The research will continue to address questions about population quality deriving from the associations of size of family of origin and sibling position with measured intelligence and the association of measured intelligence with fertility.